In the operation of combines, it is desirable to adjust operation in accordance with crop conditions and combines have been provided with infinitely variable transmissions for controlling the forward speed of travel, so that the operator may manually adjust the speed of travel, reducing the speed when moving into a region of dense crop conditions and increasing the speed when the crop is relatively thin and light. Systems have also been proposed for automatically controlling the forward speed. For example, the Andersen U.S. Pat. No. 3,073,099 discloses a combine control system wherein the drive train between the engine and the ground wheels include the pulley having a variable diameter, the transmission ratio being controlled in response to a device which measures the volume rate of flow of crop through a feeder. A modification is also disclosed in which the torque applied to a cutter bar is measured by an idler pulley engaged with a drive belt, the idler pulley being coupled to a servo valve controlling the transmission ratio.
The Pool et al. U.S. Pat. No. 3,481,122 and the Pool et al. U.S. Pat. No. 3,546,860 disclose ground speed or forward travel controls using mechanical linkages which respond to torque sensing means including spring-biased idler pulleys.
The Pitt et al. U.S. Pat. No. 3,093,946 discloses another type of arrangement in which a hydraulic motor is used to drive a threshing mechanism with the pressure in lines to the hydraulic motor being measured to measure the load of the threshing mechanism, and with an infinitely variable transmission being controlled hydraulically in response to the sensed pressure.
The Budzich U.S. Pat. No. 3,138,908 shows another control using variable diameter pulleys for driving ground wheels from an engine and a spring-loaded idler pulley engaged with a belt between the engine and a threshing mechanism, connected to a servo valve in a hydraulic system used to control the transmission ratio.
Such systems have limitations in that accurate control through hydraulic and mechanical arrangements is difficult and incorporating the controls in combines of different designs is a problem, especially in that there is usually very little physical space available for installation of additional control apparatus.
An electronic control system is proposed in the Cornish et al. U.S. Pat. No. 3,514,929 in which the torque applied to a feeder house conveyor drive is measured through the use of coaxially rotatable drive and driven elements interconnected by spring means, with the phase displacement of such elements being measured to develop a feed rate signal. The feed rate signal is combined with a feed rate reference signal to develop a signal which is compared, in turn, with a velocity set point signal and a velocity signal from a transmission transducer, the sum of which signals being used to control an infinitely variable transmission. This proposed system appears to have limitations in that it is not clear how the feed rate and velocity reference signals would be or could be adjusted in accordance with operating conditions to obtain optimum performance.